Geoscience Reference
In-Depth Information
alluvial plains, shallow soils on steep slopes, and to a lesser degree, on fertile deep
sands. Wet inceptisols (also called aquepts or gleysols by FAO) occur on older allu-
vial plains and along rivers and swamps in tropical Central and South America,
Africa, and Asia. These permanently wet soils have been used for rice cultivation
for centuries in Asia.
Other soils in the tropics include the more fertile alfisols, fluvents, lithic enti-
sols, and spodosols. There are a few other soil types found within the Tropical
Rainforest Biome, but they occur over small areas.
Soil Characteristics
Soil temperature shows even smaller variation than air temperatures in the tropical
rainforest. Ground temperature is influenced by soil moisture content as well as air
temperature. Within the root zone, about 2-20 in (5-50 cm), temperatures hardly
fluctuate. There appears to be no diurnal or annual cycles for ground temperatures
in the rainforest. These constant temperatures allow tropical plants to remain phys-
iologically active throughout the year. This is different from mid-latitude soils and
biomes where soil temperature fluctuations have a major influence on plants' meta-
bolic activity.
Soil moisture can be a determining factor in the formation and stability of the
tropical rainforests. Higher water storage capacity of the soil is crucial in tropical
rainforests, particularly where rainfall is less constant and can lead to a deficit in
the water balance of the ecosystem. Soil water storage capacity is closely related to
soil texture. Tropical soils tend to be largely clay. Clay soils tend to be low in
nutrients and fertility, and have a high water-holding capability. Superstaturation
of some of these clay soils on steeper slopes can lead to erosion and landslides. A
smaller, yet significant percentage of tropical soils are considered loams. Loamy
soils have higher nutrient and organic content than clays. These soils are more fer-
tile, and they allow for deeper root penetration and consequently fewer tree fall
hazards. Few tropical soils are sandy with the exception of smaller areas within the
Amazon and Congo basins and in the mangrove forests of the world.
Topography is an important component in soil formation and vegetative
growth in the Tropical Rainforest Biome. Flat to gently sloping hills (0-10 percent
slope angle) occupy about a third of the tropical landscape. These areas are typi-
cally well drained. A small percentage of these flat areas have soils with poor drain-
age and are swamps or flooded forests. An additional third of the tropics are rolling
hills (10-30 percent slope). The last third are steep slopes (>30 percent slope). The
steeper slopes tend to have shallow soils and are often rocky.
Nutrient Cycling and Decomposition
Tropical soils contribute little to the cycling of nutrients within the Tropical Rain-
forest Biome; however, some contribute needed phosphorus and nitrogen. The
larger contributor is the forest vegetation and organisms themselves within the
layer directly above the soil. Intense organic activity decays material dropped from
